In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.
Another way to thermally obtain a print using the electronic signals described above is to use a laser instead of a thermal printing head. In such a system, the donor sheet includes a material which strongly absorbs at the wavelength of the laser. When the donor is irradiated, this absorbing material converts light energy to thermal energy and transfers the heat to the dye in the immediate vicinity, thereby heating the dye to its vaporization temperature for transfer to the receiver. The absorbing material may be present in a layer beneath the dye and/or it may be admixed with the dye. The laser beam is modulated by electronic signals which are representative of the shape and color of the original image, so that each dye is heated to cause volatilization only in those areas in which its presence is required on the receiver to reconstruct the color of the original object. Further details of this process are found in GB 2,083,726A, the disclosure of which is hereby incorporated by reference.
In one ablative mode of imaging by the action of a laser beam, an element with a dye layer composition comprising an image dye, an infrared-absorbing material, and a binder coated onto a substrate is imaged from the dye side. The energy provided by the laser drives off substantially all of the image dye and binder at the spot where the laser beam hits the element. In ablative imaging, the laser radiation causes rapid local changes in the imaging layer thereby causing the material to be ejected from the layer. The transmission density serves as a measure of the completeness of image dye removal by the laser.
Flexographic plates, particularly those using liquid photopolymers, have a problem achieving proper highlights and shadows simultaneously with a single exposure. One method to enhance image quality for flexographic printing applications uses a tinted film process or digital masking. This method involves partially ablating a dry ablation film such as the Kodak Direct Image Recording Film to obtain a mask. This mask is used to generate a flexographic plate.
This partial ablation method uses three levels of UV transmission: unimaged (D-max), partially ablated (D-intermediate) and fully imaged (D-min) to generate a three-level mask. By controlling the three levels of UV transmission, the highlights, midtones and shadows of the flexographic plate can be adjusted independently for optimum reproduction with a single UV exposure.
Ablation films, such as the Kodak Direct Image Recording Film, are generally designed to have very high contrast. There is a problem, however, using this ablation film in the partial ablation method when trying to maintain a uniform density level at the partially ablated (D-intermediate) level. The partially ablated (D-intermediate) density varies rapidly with fluctuations in laser power, spot size, spot shape, and focus. The slope of the curve of density vs. exposure is a good measure of the film's susceptibility to these fluctuations.
U.S. Pat. No. 5,468,591 relates to a barrier layer, such as a vinyl polymer and an IR-dye, for laser ablative imaging. There is a problem using that recording element in the partial ablation method because its characteristic density vs. exposure curve does not exhibit a plateau or low slope region at intermediate exposures, so that the intermediate density level is susceptible to fluctuations in exposure.
U.S. Pat. No. 5,171,650 relates to an ablation-transfer image recording process. In that process, an element is employed which contains a dynamic release layer which absorbs imaging radiation which in turn is overcoated with an ablative carrier topcoat. Examples of the dynamic release layer include thin films of metals. An image is transferred to a receiver in contiguous registration therewith. However, this process requires the element to be exposed through the support so that there is no intermediate level of transfer possible.
It is an object of this invention to provide a method of using an ablative recording element that has a characteristic density vs. exposure curve that exhibits a plateau or low slope region at intermediate exposures, so that variations in exposure do not substantially change the partially ablated optical density level. It is another object of this invention to provide a method of using an ablative recording element that has a characteristic density vs. exposure curve that exhibits a plateau or low slope region at intermediate exposures which does not substantially decrease the speed of the recording element.